The present invention relates to the assembly of boxes loaded with printed circuit boards for electronic equipment and, more particularly, to a building box type plug-in modular rack structure for electronic equipment.
It is a common practice to provide an electronic exchange or similar electronic communication equipment with a rack structure made up of electronic units and a rack accommodating the electronic units. The electronic units are each constituted by a plurality of printed circuit boards on which various electronic parts are packaged, and a shelf or subrack accommodating the circuit boards. Alternatively, the shelf loaded with printed circuit boards may itself be configured as a single framework, and a plurality of such frameworks may be stacked one upon the other. Such conventional rack structures are disclosed in, for example, Japanese Patent Laid-Open Publication No. 211295/1982.
However, a problem with the first-mentioned conventional rack scheme is that since the electronic units each has a box-like configuration, they increase the number of parts when received in the rack having a predetermined size. Moreover, regarding communication equipment, the number of electronic units to be accommodated in a single rack increases or decreases depending on the number of terminal units to be connected to the equipment and the processing ability of such terminal units. Hence, when the number of terminal units is small or their processing ability is low, only a small number of electronic units are mounted on a single rack, wasting much of the space available in the rack. In addition, when the communication equipment is packaged and transported, the rack having substantial dimensions is the minimum unit of packaging and transport even when the number of electronic units accommodated therein is small. This is undesirable from the cost standpoint.
Even the second-mentioned rack scheme is not fully satisfactory for the following reasons. Assume that the communication equipment is scaled up and brought into operation. Then, it is extremely difficult to replace desired one of the electronic units mounted on the stacked frameworks. This is especially true when the desired electronic unit is received in any of the frameworks underlying the uppermost framework and when the electronic units are connected by, for example, cables. Furthermore, upper and lower portions of each framework have to be mechanically strong enough to sustain a plurality of shelves and, therefore, need a substantial dimension. It follows that when the frameworks are sequentially stacked, the overall rack has the dimensions thereof increased, mainly in the direction of height. This is contradictory to the increasing demand for miniature communication equipment.